How does this even happen?

Sometimes, I have good days. And sometimes, good days become good weeks. I can't go anywhere without smiling, I'm so hopeful that my attitude is starting to turn around and I'm confident that I have enough good memories and good things to look forward to so I'm mentally armed for my next bad day. Honestly nothing in particular triggers it, but if I'm left alone to think for long enough, my mind always goes to a dark place and nothing in my life sounds exciting anymore. How can a perfectly good day flip a switch in my head for no reason at all? I don't feel lonely when I'm alone (usually). Sometimes I prefer it. But inevitably my mind runs in circles when I'm alone and I end up feeling like a sad log. Advice?